Diffusion of Gases in Amorphous Polymers: The Monte Carlo Void Method Based on Molecular Dynamics; Application to He Diffusion in Polyethylene

The diffusion of small molecules in polymer matrices involves time scales (microseconds [μs] to milliseconds [ms]) far too long for routine molecular dynamics (MD) simulations. We have formulated a new method to extract coarse-grained information from short (1-2 nanoseconds [ns]) MD simulations and use this in a mesoscale simulation to calculate diffusion constants in polymer matrices. It uses a grid to calculate the average probability of each grid point of being a void and does constrained Monte Carlo (MC) dynamics to reach much longer time regimes than possible in MD. The MC method mimics the three regimes of mean square deviation (MSD) behavior seen in MD, thus accounting for the proper mobility of the voids and the compressibility of the polymer matrix. Initial results on He diffusion in a low-density polyethylene (PE) matrix are presented. The behavior at different temperatures follows closely the trend observed from calibrating long term MD for this particular system. Details of the methodology are discussed in this paper. Permanent Address: Asahi Chemical Industry Co., Ltd., 2-1 Samejima, Fuji, Shizuoka 416, Japan *Authors to whom correspondence should be sent: e-mail: [email protected] and [email protected].